There are many existing A/D and D/A converter designs for converting signals from analog to digital form and vice versa. Generally, in the conversion of a digital signal to a corresponding analog signal each bit of the digital signal is converted to a scaled analog equivalent in accordance with its bit position, and summed with all other proportionately scaled analog signals to produce a total analog signal that corresponds to the digital signal. A typical method of converting an analog signal into digital form is to successively compare the analog signal with subdivisions of a reference signal, each subdivision representing one bit and being related to other subdivisions through a power of two.
Whether the conversion is from analog-to-digital or from digital-to-analog, the converters generally require components of relatively high precision to assure adequate accuracy and resolution. If component values are not kept within a close tolerance of their nominal values, then the cumulative effect of excess tolerances will be reflected in converted signals that do not accurately correspond to the original signal. Specifically, excess component tolerances may cause an analog signal to be converted into a digital representation that is incorrect in one or more of the lower order bits and similarly may cause a digital signal to be converted into an analog signal whose magnitude corresponds to a similar but different digital signal. In summary, the accuracy of the converter is a direct function of the accuracy of the components, or, stated otherwise, the number of bits that can be handled by a converter is limited by the accuracy of the converter components.
The current thrust in electronics design is toward microelectronics, where LSI technology allows an entire circuit to be laid down in monolithic form on a single chip. If a circuit is to be produced in sufficiently large quantity, LSI technology affords significant savings in circuit size and cost. However, one recognized limitation of LSI technology is the inability to closely control the tolerances of circuit components. See in this regard, J. D. Meindl, "Microelectronic Circuit Elements", Science American, September 1977, at 70, 76. There are methods for improving the accuracy of component values, e.g. laser trimming, but these methods too have their limitations and require additional chip fabrication costs that may be even so great as to defeat the cost-effectiveness of monolithic circuitry.
The present state of the art allows D/A and A/D converters to be fabricated on an LSI chip with an upper limit of accuracy on the order of 2.sup.-10 to 2.sup.-12 of maximum value, i.e. ten to twelve bits. The general precision of LSI circuit components does not generally permit the design of converters with resolution beyond that bit level. However, there are numerous applications for converters with up to sixteen bits.
It is an objective of the present invention to realize the efficiencies of LSI technology with a converter design and associated method of operation that accommodates imprecise component values without a corresponding reduction in converter accuracy.